Throughout adult life, bone is continually undergoing remodeling through the interactive cycles of bone formation and resorption (bone turnover). Bone resorption typically is rapid, and is mediated by osteoclasts (bone resorbing cells), formed by mononuclear phagocytic precursor cells at bone remodeling sites. This process then is followed by the appearance of osteoblasts (bone forming cells) which form bone slowly to replace the lost bone. The activities of the various cell types that participate in the remodeling process are controlled by interacting systemic (e.g., hormones, lymphokines, growth factors, vitamins) and local factors (e.g., cytokines, adhesion molecules, lymphokines and growth factors). The fact that completion of this process normally leads to balanced replacement and renewal of bone indicates that the molecular signals and events that influence bone remodeling are tightly controlled.
A number of bone growth disorders are known which cause an imbalance in the bone remodeling cycle. Chief among these are metabolic bone diseases, such as osteoporosis, osteoplasia (osteomalacia), chronic renal failure and hyperparathyroidism, which result in abnormal or excessive loss of bone mass (osteopenia). Other bone diseases, such as Paget's disease, also cause excessive loss of bone mass at localized sites.
Osteoporosis is a structural deterioration of the skeleton caused by loss of bone mass resulting from an imbalance in bone formation, bone resorption, or both, such that the resorption dominates the bone formation phase, thereby reducing the weight-bearing capacity of the affected bone. In a healthy adult, the rate at which bone is formed and resorbed is tightly coordinated so as to maintain the renewal of skeletal bone. However, in osteoporotic individuals an imbalance in these bone remodeling cycles develops which results in both loss of bone mass and in formation of microarchitectural defects in the continuity of the skeleton. These skeletal defects, created by perturbation in the remodeling sequence, accumulate and finally reach a point at which the structural integrity of the skeleton is severely compromised and bone fracture is likely. Although this imbalance occurs gradually in most individuals as they age ("senile osteoporosis"), it is much more severe and occurs at a rapid rate in postmenopausal women. In addition, osteoporosis also may result from nutritional and endocrine imbalances, hereditary disorders and a number of malignant transformations.
Patients suffering from chronic renal (kidney) failure almost universally suffer loss of skeletal bone mass (renal osteodystrophy). While it is known that kidney malfunction causes a calcium and phosphate imbalance in the blood, to date replenishment of calcium and phosphate by dialysis does not significantly inhibit osteodystrophy in patients suffering from chronic renal failure. In adults, osteodystrophic symptoms often are a significant cause of morbidity. In children, renal failure often results in a failure to grow, due to the failure to maintain and/or to increase bone mass.
Osteoplasia, also known as osteomalacia ("soft bones"), is a defect in bone mineralization (e.g., incomplete mineralization), and classically is related to vitamin D deficiency (1,25-dihydroxy vitamin D.sub.3). The defect can cause compression fractures in bone, and a decrease in bone mass, as well as extended zones of hypertrophy and proliferative cartilage in place of bone tissue. The deficiency may result from a nutritional deficiency (e.g., rickets in children), malabsorption of vitamin D or calcium, and/or impaired metabolism of the vitamin.
Hyperparathyroidism (overproduction of the parathyroid hormone) is known to cause malabsorption of calcium, leading to abnormal bone loss. In children, hyperparathyroidism can inhibit growth, in adults the skeleton integrity is compromised and fracture of the ribs and vertebrae are characteristic. The parathyroid hormone imbalance typically may result from thyroid adenomas or gland hyperplasia, or may result from prolonged pharmacological use of a steroid. Secondary hyperparathyroidism also may result from renal osteodystrophy. In the early stages of the disease osteoclasts are stimulated to resorb bone in response to the excess hormone present. As the disease progresses, the trabecular bone ultimately is resorbed and marrow is replaced with fibrosis, macrophages and areas of hemorrhage as a consequence of microfractures. This condition is referred to clinically as osteitis fibrosa.
Paget's disease (osteitis deformans) is a disorder currently thought to have a viral etiology and is characterized by excessive bone resorption at localized sites which flare and heal but which ultimately are chronic and progressive, and may lead to malignant transformation. The disease typically affects adults over the age of 25.
To date, osteopenia treatments are based on inhibiting further bone resorption, e.g., by 1) inhibiting the differentiation of hemopoietic mononuclear cells into mature osteoclasts, 2) by directly preventing osteoclast-mediated bone resorption, or 3) by affecting the hormonal control of bone resorption. Drug regimens used for the treatment of osteoporosis include calcium supplements, estrogen, calcitonin and diphosphonates. Vitamin D.sub.3 and its metabolites, known to enhance calcium and phosphate absorption, also are being tried. None of the current therapies stimulate regeneration of new bone tissue. In addition, all of these agents have only a transient effect on bone remodeling. Thus, while in some cases the progression of the disease may be halted or slowed, patients with significant bone deterioration remain actively at risk. This is particularly prevalent in disorders such as osteoporosis where early diagnosis is difficult and/or rare and significant structural deterioration of the bone already may have occurred.
It is an object of the present invention to develop methods and compositions for inhibiting or preventing the loss of bone mass and/or for increasing bone formation in an individual who, for example, is afflicted with a disease which decreases skeletal bone mass, particularly where the disease causes an imbalance in bone remodeling. Another object is to enhance bone growth in children suffering from bone disorders, including metabolic bone diseases. Still another object is to prevent or inhibit bone deterioration in individuals at risk for loss of bone mass, including postmenopausal women, aged individuals, and patients undergoing dialysis. Yet another object is to provide methods and compositions for repairing defects in the microstructure of structurally compromised bone, including repairing bone fractures. Thus, the invention is aimed at stimulating bone formation and increasing bone mass, optionally over prolonged periods of time, and particularly to decrease the occurrence of new fractures resulting from structural deterioration of the skeleton. These and other objects and features of the invention will be apparent from the description, drawings, and claims which follow.